State who told sheriffbot to roll the Chromium deps and to what revision
https://bugs.webkit.org/show_bug.cgi?id=106626

Patch by Alan Cutter <alancutter@chromium.org> on 2013-01-13
Reviewed by Eric Seidel.

Added optional argument to post-chromium-deps-roll so Sheriffbot can specify ChangeLog message.

* Scripts/webkitpy/tool/bot/irc_command.py:
(RollChromiumDEPS._expand_irc_nickname):
(RollChromiumDEPS.execute):
* Scripts/webkitpy/tool/bot/irc_command_unittest.py:
(IRCCommandTest.test_roll_chromium_deps):
* Scripts/webkitpy/tool/bot/sheriff.py:
(Sheriff.post_chromium_deps_roll):
* Scripts/webkitpy/tool/commands/roll.py:
(RollChromiumDEPS._prepare_state):
(PostChromiumDEPSRoll):
(PostChromiumDEPSRoll._prepare_state):
* Scripts/webkitpy/tool/commands/roll_unittest.py:
(PostRollCommandsTest.test_prepare_state):
* Scripts/webkitpy/tool/steps/preparechangelogfordepsroll.py:
(PrepareChangeLogForDEPSRoll.run):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139578 268f45cc-cd09-0410-ab3c-d52691b4dbfc